Method for the continuous production of epoxids from olefins and hydroperoxides on a suspended catalyst

ABSTRACT

Continuous process for the epoxidation of olefins by means of hydroperoxide, in which the epoxidation is carried out in a reactor in which at least one catalyst suspended in a liquid phase is present, wherein the liquid phase is passed through a device which has openings or channels and is installed in the reactor and the epoxide-containing liquid is separated off by means of crossflow filtration so that the suspended catalyst is retained in the reaction system.

The present invention relates to a continuous epoxidation process forconverting olefins into epoxides in a reactor in which at least onecatalyst suspended in a liquid phase and, if desired, additionally a gasphase are present, wherein the liquid phase and, if present, the gasphase are passed through a device having openings or channels in thereactor and the epoxide-containing liquid is separated off by means of acrossflow filtration so that the suspended catalyst is retained in thereaction system. The invention also relates to an apparatus for carryingout the process. Process and apparatus are preferably used in theepoxidation of propene by means of hydrogen peroxide to form propeneoxide.

According to the prior art, the epoxidation of olefins by means ofhydroperoxide can be carried out in one or more stages, with both batchprocesses and continuous processes being possible. The epoxidation ispreferably also catalyzed, either in a heterogeneous or homogeneousphase. Processes are described, for example, in WO 00/07965.

Use of a fixed-bed reactor to carry out the heterogeneously catalyzedepoxidation is also known. For this purpose, specially preparedcatalysts usually have to be produced. In such a use, the catalyst ispreferably applied to support materials or processed to form specificshaped bodies. However, when the activity drops, which may occur afteronly relatively short periods of operation, the catalyst can often beremoved from the fixed bed or regenerated only with some difficulty.This is usually associated with a shutdown of the entire plant, i.e. notonly the epoxidation stage but also but also the following work-upstage. This leads to a low space-time yield, which is disadvantageousfor an industrial process.

It is an object of the present invention to develop a process for theepoxidation of olefins by means of hydroperoxides, in which the catalystcan easily be replaced during the reaction without shutdown of the plantbeing necessary, while at the same time achieving a high space-timeyield.

We have found that this object is achieved by a continuous process forthe epoxidation of olefins, in which the epoxidation is carried out in areactor in which at least one catalyst suspended in a liquid phase ispresent, wherein the liquid phase is passed through a device which hasopenings or channels and is installed in the reactor and theepoxide-containing liquid is separated off by means of crossflowfiltration so that the suspended catalyst is retained in the reactionsystem.

If a gas phase is present, this too can be passed through the devicewhich has openings or channels and is installed in the reactor.

The device having openings or channels through which the reaction mediumis passed can comprise a bed, a knitted mesh or a packing element. Suchdevices are known from distillation and extraction technology.

However, for the purposes of the present invention, such devices inprinciple have a substantially smaller hydraulic diameter than thedevices used as internals in distillation and extraction technology. Inthe novel process, this diameter is preferably smaller by a factor offrom 2 to 10. The hydraulic diameter of the device used as internal inthe reactor in the process of the present invention is preferably from0.5 to 20 mm.

The hydraulic diameter is a characteristic quantity for the descriptionof the equivalent diameter of non-circular openings or channelstructures.

In the context of the present invention, the term “hydraulic diameter”relates to the ratio of four times the cross-section of the opening andthe circumference of the opening. In case a channel structure having across-section in the shape of an isosceles triangle is concerned, theterm “hydraulic diameter” relates to the quantity 2bk/(b+2s) wherein bis the length of the basis, k is the height and s is the length of thelateral side of the triangle.

Packing elements which offer the advantage of a low pressure drop are,for example, woven wire mesh packings. Apart from woven mesh packings,it is also possible to use packings comprising other woven, knitted orfelted liquid-permeable materials.

Further suitable packings or packing elements which can be used are flatmetal sheets, preferably without perforation or other relatively largeopenings. Examples are commercial types such as B1 from Montz orMellapak from Sulzer.

Packings made of expanded metal, for example BSH packing from Montz, arealso advantageous. Here too, openings which are, for instance, in theform of perforations have to be kept appropriately small. The decisivefactor determining the suitability of packing for the purposes of thepresent invention is not its geometry but the widths of openings orchannels in the packing which allow flow to occur.

To suspend the solid particles in the reactor, mechanical energy isintroduced into the reactor, preferably by means of stirrers, nozzles orrising gas bubbles. The installation of the abovementioned devices inthe reactor produces an increases difference in the motion of thecatalyst particles relative to the liquid phase in the reaction section,since the particles are held back more strongly than the surroundingliquid in the narrow openings and channels of these devices. Thisincreased relative velocity improves mass transfer between liquid andsuspended particles, which is important for achieving a high space-timeyield.

The use of catalyst particles having particle sizes in the range from 1to 10 mm for suspension catalysts is also known. Although particles ofthis size have the desired relative velocity relative to the surroundingliquid, their low surface area per unit volume limits turnover. The twoeffects frequently cancel out one another, so that the problem ofincreasing mass transport is not solved in the final analysis.

In contrast thereto, the catalyst particles used in the process of thepresent invention preferably have a mean particle size of from 0.0001 to2 mm, more preferably from 0.0001 to 1 mm, particularly preferably from0.005 to 0.1 mm. Particles of this mean particle size surprisinglyenable the relative velocity and mass transport to be increased further.

In the novel process, the high relative velocity which can be achievedis also extremely advantageous compared to processes in which reactorswithout the abovementioned internals are used. Increasing theintroduction of mechanical energy above that required for achievingsuspension leads to no appreciable improvement in mass transfer betweenthe liquid and the suspended solid particles in suspension reactorswithout internals, since the relative velocity which can be achieved isonly insignificantly higher than the sedimentation velocity.

When the internals in the reactor are combined with catalyst particlesin the particle size range indicated, high relative velocities of theliquid phase relative to the catalyst particles and thus advantageousmass transport are achieved. The novel process is therefore superior toprocesses in which no internals are used in the reactor or catalystparticles having a greater diameter are used.

The process can be carried out in various continuously operated types ofreactor, e.g. jet nozzle reactors, bubble columns or shell-and-tubereactors. It is not necessary for the internals to fill the entirereactor.

Particularly preferred embodiments of the reactor are bubble columns orshell-and-tube reactors.

A very particularly preferred reactor is a heatable and coolableshell-and-tube reactor in which the internals are accommodated in theindividual tubes. Such a reactor has the advantage that the energyrequired for activation of the reaction can be readily introduced or theheat of reaction evolved can be readily removed.

Preference is given to the reactor being arranged vertically and thereaction mixture flowing through it from the bottom upward.

In the process of the present invention, the epoxidation is carried outin a reactor having one of the above-described internals in the presenceof one or more suspension catalysts at a pressure of from 1 to 100 bar,preferably from 1 to 60 bar, particularly preferably from 1 to 50 bar.The reaction temperature is in the range from 20 to 100° C., preferablyfrom 30 to 80° C., particularly preferably from 40 to 70° C.

The process is simple to carry out. The above-described device,preferably woven mesh packing or sheet metal packing, is installed inthe reactor. The reaction mixture comprising olefin, hydroperoxide andsuspension catalyst is then circulated at high velocity through thereactor by means of a pump. The throughput per unit cross-sectional area(empty tube velocity) of the liquid phase is preferably from 50 to 300m³/m² h, in particular in the range from 100 to 250 m³/m² h.

The suspended catalyst material is introduced into the reactor with theaid of customary techniques. Retention of the suspension catalyst in thereaction system while the epoxide-containing liquid phase is separatedoff is achieved by the use of crossflow filtration.

Membranes suitable for the crossflow filtration are specifically treatedaluminum oxide or sintered metal membranes having pore diameters of from50 to 500 nm, preferably from 50 to 100 nm, as are marketed by, forexample, Membraflow. The membrane modules, in general multichannelmodules, are installed in the reaction circuit in such a way that theflow velocity in the individual channels is from 1 to 6 m/s, preferablyfrom 2 to 4 m/s, and no deposit can settle on the membrane surfaces as aresult. The permeate stream, i.e. the epoxide-containing liquid streamwhich passes through the membrane, is taken off perpendicular to themain flow direction. The amount is regulated via the prevailingtrans-membrane pressure. A trans-membrane pressure in the range from 0.2to 2 bar, preferably from 0.3 to 1 bar, is desirable. The trans-membranepressure is defined as the difference between the mean pressure on thefeed or retentate side and the pressure on the permeate side.

The epoxide-containing liquid is obtained as permeate and can be passedto work-up.

If the activity of the catalyst drops to such an extent that the processproceeds only unsatisfactorily, it can be conveniently separated off thesystem, replaced or regenerated. Preference is given to part of thecatalyst suspension being discharged from the system during the reactionand being replaced by fresh catalyst suspension. The deactivatedcatalyst can then be regenerated externally. Interruption of theepoxidation stage or the work-up stage of the epoxide-containing liquidis thus not necessary, which is extremely advantageous.

In the process, the epoxide-containing solution is replaced by startingmaterials and solvent in the amount corresponding to that in which thesolution is taken off. This makes a continuously operated processpossible, which is extremely useful for industrial implementation.

The starting materials known from the prior art can be used for theepoxide synthesis in the process of the present invention.

Preference is given to using organic compounds which have at least oneC—C double bond. Examples of such organic compounds having at least oneC—C double bond are the following alkenes:

ethene, propene, 1-butene, 2-butene, isobutene, butadiene, pentenes,piperylene, hexenes, hexadienes, heptenes, octenes, diisobutene,trimethylpentene, nonenes, dodecene, tridecene, tetradecenes toeicosenes, tripropene and tetrapropene, polybutadienes, polyisobutenes,isoprenes, terpenes, geraniol, linalool, linalyl acetate,methylenecyclopropane, cyclopentene, cyclohexene, norbornen,cycloheptene, vinylcyclohexane, vinyloxirane, vinylcyclohexene, styrene,cyclooctene, cyclooctadiene, vinylnorbomene, indene, tetrahydroindene,methylstyrene, dicyclopentadiene, divinylbenzene, cyclododecene,cyclododecatriene, stilbene, diphenylbutadiene, vitamin A,beta-carotene, vinylidene fluoride, allyl halides, crotyl chloride,methallyl chloride, dichlorobutene, allyl alcohol, methallyl alcohol,butenols, butenediols, cyclopentenediols, pentenols, octadienols,tridecenols, unsaturated steroids, ethoxyethene, isoeugenol, anethole,unsaturated carboxylic acids such as acrylic acid, methacrylic acid,crotonic acid, maleic acid, vinylacetic acid, unsaturated fatty acidssuch as oleic acid, linoleic acid, palmitic acid, naturally occurringfats and oils.

Particular preference is given to using alkenes which contain from 2 to8 carbon atoms, e.g. ethene, propene and butene.

Very particular preference is given to using propene.

It is also possible to use “chemical grade” propene. In this case,propene is present together with propane in a volume ratio of propene topropane of from about 97:3 to 95:5.

As hydroperoxides, it is possible to use the known hydroperoxides whichare suitable for the reaction of the organic compound. Examples of suchhydroperoxides are tert-butyl hydroperoxide or ethylbenzenehydroperoxide. Hydrogen peroxide is preferably used as hydroperoxide forthe epoxide synthesis, preferably as an aqueous hydrogen peroxidesolution.

As heterogeneous catalysts, use is made of ones which comprise a porousoxidic material, e.g. a zeolite. Preference is given to using catalystswhich comprise a titanium-, germanium-, tellurium-, vanadium-,chromium-, niobium- or zirconium-containing zeolite as porous oxidicmaterial.

Specific examples are titanium-, germanium-, tellurium-, vanadium-,chromium-, niobium-, zirconium-containing zeolites having a pentasilzeolite structure, in particular the types which can be assigned X-raycrystallographically to the ABW, ACO, AEI, AEL, AEN, AET, AFG, AFI, AFN,AFO, AFR, AFS, ATF, AFX, AFY, AHT, ANA, APC, APD, AST, ATN, ATO, ATS,ATT, ATV, AWO, AWW, BEA, BIK, BOG, BPH, BRE, CAN, CAS, CFI, CGF, CGS,CHA, CFI, CLO, CON, CZP, DAC, DDR, DFO, DFF, DOH, DON, EAB, EDI, EMT,EPI, ERI, ESV, EUO, FAU, FER, GIS, GME, GOO, HEU, IFR, ISV, IFE, JBW,KFI, LAU, LEV, LIO, LOS, LOV, LTA, LTL, LTN, MAZ, MEI, MEL, MEP, MER,MFI, MFS, MON, MOR, MSO, MTF, MTN, MTT, MTW, MWW, NAT, NES, NON, OFF,OSI, PAR, PAU, PHI, RHO, RON, RSN, RTE, RTH, RUT, SAO, SAT, SBE, SBS,SBT, SFF, SGT, SOD, STF, STI, STT, TER, THO, TON, TSC, VET, VFI, VNI,VSV, WIE, WEN, YUG, ZON structure or to mixed structures of two or moreof the abovementioned structures. The use of titanium-containingzeolites having the ITQ-4, SSZ-24, TTM-1, UTD-1, CIT-1 or CIT-5structure is also conceivable in the process of the present invention.Further titanium-containing zeolites which may be mentioned are thosehaving the ZSM-48 or ZSM-12 structure.

Particular preference is given to Ti zeolites having the MFI or MELstructure or the MFI/MEL mixed structure. Very particular preference isgiven to the titanium-containing zeolite catalysts which are generallyreferred to as “TS-1”, “TS-2” and “TS-3”, and also Ti zeolites having alattice structure isomorphous with β-zeolite.

The use of a heterogeneous catalyst comprising the titanium-containingsilicalite TS-1 is very advantageous.

It is possible, inter alia, to use the porous oxidic material itself ascatalyst. However, it is also possible to use a shaped body comprisingthe porous oxidic material as catalyst. To produce the shaped body fromthe porous oxidic material, it is possible to employ all processes knownfrom the prior art.

In these processes, noble metals can be applied in the form of suitablenoble metal components, for example in the form of water-soluble salts,to the catalyst material before, during or after one or more shapingsteps. This method is preferably employed for producing oxidationcatalysts based on titanium silicates or vanadium silicates having azeolite structure, and makes it possible to obtain catalysts having acontent of from 0.01 to 30% by weight of one or more noble metals fromthe group consisting of ruthenium, rhodium, palladium, osmium, iridium,platinum, rhenium, gold and silver. Such catalysts are described, forexample, in DE-A 196 23 609.6.

Of course, the shaped bodies can be subjected to finishing treatment.All methods of comminution, for example milling, splitting or crushingof the shaped bodies, and also further chemical treatments as describedby way of example above are conceivable.

When using a shaped body or a plurality thereof as catalyst, this can,after it has been deactivated, be regenerated in the process of thepresent invention by a method in which regeneration is achieved bytargeted burning-off of the deposits responsible for deactivation. Thisis preferably carried out in an inert gas atmosphere containingprecisely defined amounts of oxygen-donating substances. Thisregeneration process is described in DE-A 197 23 949.8. It is alsopossible to use the regeneration processes cited there in the discussionof the prior art.

As solvents, preference is given to using all solvents which completelyor at least partly dissolve the starting materials used in the epoxidesynthesis. For example, it is possible to use water; alcohols,preferably lower alcohols, more preferably alcohols having less than 6carbon atoms, for example methanol, ethanol, propanols, butanols,pentanols, diols or polyols, preferably those having less than 6 carbonatoms; ethers such as diethyl ether, tetrahydrofuran, dioxane,1,2-diethoxyethane, 2-methoxyethanol; esters such as methyl acetate orbutyrolactone; amides such as dimethylformamide, dimethylacetamide,N-methylpyrrolidone; ketones such as acetone; nitrites such asacetonitrile; sulfoxides such as dimethyl sulfoxide; aliphatic,cycloaliphatic and aromatic hydrocarbons, or mixtures of two or more ofthe abovementioned compounds.

Preference is given to using alcohols. Here, the use of methanol assolvent is particularly preferred.

In the reaction of the olefin with the hydroperoxide, it is alsopossible for further compounds which are customarily used in epoxidationreactions to be present. Such compounds are, for example, buffers bymeans of which the pH range favorable for the respective epoxidation canbe set and the activity of the catalyst can be regulated.

The invention further provides an apparatus for carrying out acontinuous process for the epoxidation of olefins by means ofhydroperoxide as is described above, comprising a reactor in which theepoxidation is carried out, a crossflow filter for separating offepoxide-containing solution so that the catalyst is retained in thereactor and a container for the catalyst suspension.

In particular, the apparatus for carrying out a continuous process forthe epoxidation of olefins comprises a reactor having internals selectedfrom the group consisting of beds, knitted meshes or packing elementsand having a hydraulic diameter of from 0.5 to 20 mm, a catalyst havinga mean particle size of from 0.0001 to 2 mm suspended in a liquid, acrossflow filter and a container for the catalyst suspension.

In a particularly preferred embodiment of the apparatus for carrying outthe process, the reactor is a bubble column or a shell-and-tube reactor.In a very particularly preferred embodiment, the reactor is ashell-and-tube reactor which makes heat removal possible.

A reactor for the epoxidation of olefins will now be described by way ofexample with the aid of FIG. 1. In such a reactor, preference is givento reacting propene with hydrogen peroxide as epoxidizing agent inmethanol as solvent using a suspended TS-1 catalyst and, if appropriate,buffer additives for controlling the reactivity of the catalyst and thepH to give propene oxide.

FIG. 1 shows, by way of example, the experimental structure of acontinuously operated reactor 1, e.g. a bubble column or particularlypreferably a heatable and coolable shell-and-tube reactor, which isprovided with heatable packings 2 and which is supplied via the lines 3with a liquid mixture comprising the olefin, hydrogen peroxide, thesolvent and, if appropriate, buffer additives. The pump 4 maintains thecirculation and thus keeps the catalyst in suspension. After leaving thereactor 1, the reaction solution is conveyed via line 5 to the crossflowfilter 6. The permeate is taken off perpendicular to the main flowdirection and is passed via the line 7 to the work-up stage of theplant.

Since the catalyst cannot pass the crossflow filter, it remainssuspended in the reactor system and is conveyed via line 8 and, ifappropriate, the heat exchanger 9 to the reactor 1, thus closing thecatalyst circuit.

Introduction or discharge of the catalyst is carried out, for example,via a container 10 which can be incorporated in a specific fashion inthe reaction circuit. To introduce catalysts, a particular amount ofcatalyst is, for example, placed in the container and the latter isfilled with solvent. The valves 11 and 12 are subsequently opened andthe valve 13 is closed. In this state, all the reaction medium flowsthrough the container 10 and the catalyst is carried into the system.

A similar procedure is used to discharge catalyst. The container 10 isfilled, for example, with methanol and the valves 11 and 12 aresubsequently opened and the valve 13 is closed. The reaction medium onceagain flows through the reactor. After the catalyst concentrations inthe reactor and the container have become equal, the valves 11 and 12are closed and the valve 13 is opened. The container 10 is now isolatedfrom the reaction medium and contains an aliquot of catalyst. This canthen be separated from the solution in a further step and possibly beregenerated externally. After regeneration, it can be fed back into thesystem as described above.

Via valve 15, catalyst material can be introduced in container 10.

LIST OF REFERENCE NUMERALS FOR FIG. 1

-   1 Reactor (bubble column, shell-and-tube reactor)-   2 Packings-   3 Feed line-   4 Pump-   5 Line-   6 Crossflow filter-   7 Line for the permeate-   8 Line-   9 Heat exchanger-   10 Container for catalyst suspension-   11 Valve-   12 Valve-   13 Valve-   14 Catalyst material-   15 Valve-   16 Valve

1-7. (canceled)
 8. A continuous process for the epoxidation of olefinsby means of hydroperoxide, wherein the epoxidation is carried out in areactor in which at least one catalyst suspended in a liquid phase ispresent in the form of particles having a mean particle size of from0.0001 to 2 mm, and wherein the liquid phase is passed through a devicewhich has openings or channels and is installed in the reactor and thecatalyst is retained in the reaction system by means of crossflowfiltration when the epoxide containing liquid is separated off, whereinthe crossflow filtration is carried out using membrane modules installedin the reaction circuit in such a way that the flow velocity in theindividual channels is from 1 to 6 m/s and wherein catalyst suspensionis taken from or fed into the reactor during the epoxidation.
 9. Aprocess as claimed in claim 8, wherein a gas phase which is present inthe reactor is also passed through the device which has openings orchannels and is installed in the reactor.
 10. A process as claimed inclaim 8, wherein the hydraulic diameter of the device installed in thereactor is from 0.5 to 20 mm.
 11. A process as claimed in claim 9,wherein the hydraulic diameter of the device installed in the reactor isfrom 0.5 to 20 mm.
 12. A process as claimed in claim 8, wherein thedevice installed in the reactor is a bed, a knitted mesh or a packingelement.
 13. A process as claimed in claim 9, wherein the deviceinstalled in the reactor is a bed, a knitted mesh or a packing element.14. A process as claimed in claim 10, wherein the device installed inthe reactor is a bed, a knitted mesh or a packing element.
 15. A processas claimed in claim 8, wherein the reactor is a jet nozzle reactor, abubble column or a shell-and-tube reactor.
 16. A process as claimed inclaim 9, wherein the reactor is a jet nozzle reactor, a bubble column ora shell-and-tube reactor.
 17. A process as claimed in claim 10, whereinthe reactor is a jet nozzle reactor, a bubble column or a shell-and-tubereactor.
 18. A process as claimed in claim 8, wherein the epoxidation iscarried out at a temperature of from 20 to 100° C. and a pressure offrom 1 to 100 bar.
 19. A process as claimed in claim 9, wherein theepoxidation is carried out at a temperature of from 20 to 100° C. and apressure of from 1 to 100 bar.
 20. A process as claimed in claim 10,wherein the epoxidation is carried out at a temperature of from 20 to100° C. and a pressure of from 1 to 100 bar.
 21. A process as claimed inclaim 8, wherein propene is epoxidized by means of hydrogen peroxideover a titanium-containing zeolite.
 22. A process as claimed in claim 9,wherein propene is epoxidized by means of hydrogen peroxide over atitanium-containing zeolite.
 23. A process as claimed in claim 10,wherein propene is epoxidized by means of hydrogen peroxide over atitanium-containing zeolite.
 24. A continuous process for theepoxidation of olefins by means of hydroperoxide, wherein theepoxidation is carried out in a reactor in which at least one catalystsuspended in a liquid phase is present in the form of particles having amean particle size of from 0.0001 to 2 mm, and wherein the liquid phaseis passed through a device which has openings or channels and isinstalled in the reactor and the catalyst is retained in the reactionsystem by means of crossflow filtration when the epoxide containingliquid is separated off, wherein the crossflow filtration is carried outusing membrane modules installed in the reaction circuit in such a waythat the flow velocity in the individual channels is from 1 to 6 m/s,wherein catalyst suspension is taken from or fed into the reactor duringthe epoxidation and wherein a gas phase which is present in the reactoris also passed through the device which has openings or channels and isinstalled in the reactor.
 25. A continuous process for the epoxidationof olefins by means of hydroperoxide, wherein the epoxidation is carriedout in a reactor in which at least one catalyst suspended in a liquidphase is present in the form of particles having a mean particle size offrom 0.0001 to 2 mm, and wherein the liquid phase is passed through adevice which has openings or channels and is installed in the reactorand the catalyst is retained in the reaction system by means ofcrossflow filtration when the epoxide containing liquid is separatedoff, wherein the crossflow filtration is carried out using membranemodules installed in the reaction circuit in such a way that the flowvelocity in the individual channels is from 1 to 6 m/s, wherein catalystsuspension is taken from or fed into the reactor during the epoxidationand wherein propene is epoxidized by means of hydrogen peroxide over atitanium-containing zeolite.
 26. A continuous process for theepoxidation of olefins by means of hydroperoxide, wherein theepoxidation is carried out in a reactor in which at least one catalystsuspended in a liquid phase is present in the form of particles having amean particle size of from 0.0001 to 2 mm, and wherein the liquid phaseis passed through a device which has openings or channels and isinstalled in the reactor and the catalyst is retained in the reactionsystem by means of crossflow filtration when the epoxide containingliquid is separated off, wherein the crossflow filtration is carried outusing membrane modules installed in the reaction circuit in such a waythat the flow velocity in the individual channels is from 1 to 6 m/s,wherein catalyst suspension is taken from or fed into the reactor duringthe epoxidation, wherein a gas phase which is present in the reactor isalso passed through the device which has openings or channels and isinstalled in the reactor and wherein the hydraulic diameter of thedevice installed in the reactor is from 0.5 to 20 mm.
 27. A continuousprocess for the epoxidation of olefins by means of hydroperoxide,wherein the epoxidation is carried out in a reactor in which at leastone catalyst suspended in a liquid phase is present in the form ofparticles having a mean particle size of from 0.0001 to 2 mm, andwherein the liquid phase is passed through a device which has openingsor channels and is installed in the reactor and the catalyst is retainedin the reaction system by means of crossflow filtration when the epoxidecontaining liquid is separated off, wherein the crossflow filtration iscarried out using membrane modules installed in the reaction circuit insuch a way that the flow velocity in the individual channels is from 1to 6 m/s, wherein catalyst suspension is taken from or fed into thereactor during the epoxidation, wherein the hydraulic diameter of thedevice installed in the reactor is from 0.5 to 20 mm and wherein propeneis epoxidized by means of hydrogen peroxide over a titanium-containingzeolite.